docs: def export_to_file(request):
      """
      Export a part of this source site's page tree to a JSON file
      on this user's filesystem for subsequent import in a destination
      site's Wagtail Admin
      """
      if request.method == 'POST':
          form = ExportForm(request.POST)
          if form.is_valid():
              payload = export_pages(form.cleaned_data['root_page'], export_unpublished=True)
              response = JsonResponse(payload)
              response['Content-Disposition'] = 'attachment; filename="export.json"'
              return response
      else:
          form = ExportForm()
  
      return render(request, 'wagtailimportexport/export_to_file.html', {
          'form': form,
      })